Solid state displays are finding wider and wider applications in the electronics and consumer industries. Liquid crystal displays have largely superseded earlier light emitting diode displays, on account of factors such as low current drain, reliability, readability and aesthetic appearance.
LCDs operate in either reflective mode, transmissive mode or transflective mode. A reflective mode LCD requires ambient light to be reflected from the display to show the contrasting segments. A transmissive mode LCD has a backlight and passes the light through the display. A transflective mode LCD has a low backlight and a thin reflective layer in the back polarizer. Transflective mode LCDs are usable in all conditions of ambient light. The ruggedness of such devices is in part dependent on the reliability of the backlights.
Positive transmissive or Transflective mode LCDs are displays in which the characters appear dark in contrast to a light background. Negative transmissive or transflective mode LCDs allow light to pass through the characters and block the passage of light through the background.
LCDs are made from a sandwich of two layers of cell with polarizable liquid between them. The liquid selectively rotates the light passing through. Front and back polarizers polarize the light and filter out light that has been rotated by the liquid, or has not been rotated, depending on the transmissive mode of the display. The cell sandwich is typically known as twisted neumatic cell and typically rotates the light through 180.degree.. Super twisted nematic cell is similar but has a liquid that imparts about 270.degree. rotation. Double super twisted nematic (DSTN) cell is a double sandwich of super twisted nematic cell where the extra sandwich is a compensation cell and gives a wider viewing angle. Normal super twisted nematic displays read to change colour with temperature. In a DSTN display, the compensation cell also gives colour compensation. DSTN displays are neutral regarding colour.
It is known to provide negative transmission mode LCD displays using DSTN cell with a cold cathode tube to provide the backlighting, generally provided via a light pipe from outside the display. Such an arrangement is only possible in such applications as aviation displays where there is adequate room for the bulky cold cathode tube and the light pipe. Cold cathode tubes are not suitable for mobile radios because of the need for a high voltage transformer, shielding and because of r.f. noise generated. A further problem with cold cathode tubes is that they cannot start below about -10.degree. C. This is acceptable in aircraft cockpits and some motor vehicle dashboards, but is not acceptable for radios which need to be operated outdoors. Ordinary filament bulbs can be used, but these have a very short lifetime.
It is also known, for transflective mode LCDs, to provide a module of LEDs to provide backlighting. Such a module comprises an array of chip LEDs mounted on a printed circuit board (PCB) and covered by a diffuser. The diffuser diffuses the light from the array of LEDs and provides uniform distribution of light across the module. The diffuser reduces the light from the LEDs by about 20%. A problem when attempting to use a module of LEDs and a diffuser as a backlight for a transmission mode LCD is that the overall display has a very thick profile. This is unacceptable in many applications, creating the same problem as the cold cathode tube arrangement.
Elektronik vol. 40 no. 19 September 1991 pages 96-99 G. Waitl et al confirms that ready-made modules are relatively inflexible for these purposes and proposes discrete "Argus" LEDs or 3 mm nondiffuse LEDs with light boxes or fine light guides. The LEDs are through-mounted into the underlying printed circuit board or other mounting. Each light box has a reflector surrounding the LED and covered by a diffusing screen. The heights of the reflectors are 9.5-17 mm to which must be added the thickness of the diffusing screen to give the minimum thickness of the backlight array. Different possibilities for rear illumination of liquid crystal displays are also shown with a transfusor between the backlight illumination source and the LCD cell.
EP-A-0363700 shows a one-dimensional row of through-mounted LEDs mounted in an elongate box having reflective areas which extend at an angle to collect the light in such a way as to uniformly illuminate a positive transmissive mode LCD display.
Technical Digest--Western Electric no. 67, July 1982 pages 1-2 shows a LCD module having a display element and a backlight sub-assembly where the display element is mounted over and directly adjacent to the backlight sub-assembly and the backlight sub-assembly comprises an array of LEDs mounted on a circuit board. Each LED has an epoxy dome to "increase the light-emitting efficiency". The modular construction makes the design inflexible. The circuit board of the module must be mounted on another circuit board for connection to other operative elements of the product of which the module forms a part, This gives a high overall thickness and does not facilitate good heat dissipation.
There is currently no low-profile display that meets all the customer requirements of viewing angle, character definition, temperature range, durability, operating life and aesthetic appearance (including colour stability).
There is a need for an improved low profile display.